Preparation of cellulose derivatives



Feb. 2 7, 1940.

J. FLEI SCHER ET AL 7 2,191,525 PREPARATION OF CELLULOSE DERIVATI VES Filed May 5, le s? INVENTQRS JOSEPH FLE/SCHE/Y ATTORNEYS Patented Feb. 27, 1940 PATENT OFFICE PREPARATIQN OF CELLULOSE DERIVATIVE Joseph Fleischer, Clifton Terrace, 111., and William A. Bailey, Jr., Baltimore, Md., assignors to The Cellulose Research Corporation, East Alton, 11].,

a corporation of Delaware Application May 5, 1937, Serial No. 140,829

.7 Claims. (01. 260-227) This invention relates to the manfrtacture of cellulose acetate and more particular y to the dehydration of cellulose prior to the acetylation thereof; 7

An object of this invention is to provide an acetylation process by means of which commercially valuable cellulose acetate may be produced from cellulosic materials such as purified wood pulp,

Another object of thisiinvention is toprovide an economical process for the dehydration and subsequent acetylation of cellulose.

A. further object of this invention is to provide a process for the dehydration of cellulose by a distillation method whereby moisture can be. removed without substantial shrinkage of the fibers and whereby resinous impurities may be extractedfrom the fibers alt thesame time,

Another object of this invention is to provide a process for dehydrating cellulose by distillation whereby the dehydrated cellulose may be directly acetylated without the customary pretreatment withacetic acid.

Another object of this invention is to provide a'process for the dehydration, prior to acety1ation, of cellulose, initially in a water-saturated condition, wherein the moisture is removed by distillation while the direct vaporization of moisture from the-fibers is avoided.

In the commercial processes for the manufacture of cellulose acetate, the initial treatment has consisted in drying purified cotton linters,

which have almost exclusively constituted the means of hot air.

starting cellulose material for this purpose, to a moisture content of about 2% to 5%,'usually by The removal. of moisture is I essential since it would otherwise react with and cause a wasteful usage of the acetylatingreagent, usually acetic anhydride, which is the most expensive raw material employed in the process. However, the drying as conventionally carried out is accompanied by disadvantageous effects,

which have been recognized and referred to in the art as loss of reactivity of the fibers. This loss of reactivity is undoubtedly connected withand probably largely due to the shrinkage of the fibers on drying, which is so extensive as tocause a considerable decrease in their crosssectional area and the almost complete closure of the lumina The shrinkage in external dimensions is evidently accompanied by a large decrease in the area of cellulose membranes within the fibers which is available or contact with treating liquids such as an acetylation mixture. It has therefore been general practice in the commercial acetylation processes of the prior art to provide a pretreatment of the dried linters with acetic acid, with or without a catalyst, for an extended period, usually of 4 to 6 hours duration, at an elevated temperature prior to the addition of 'the'active acetylating reagent, in order to improve this condition.

The lengthening of the acetylation process is an obvious disadvantage of the above procedure. Furthermore, it is properly operative for purified cotton linters only when the evaporation of mois- Numerous attempts have been made in the past to substitute purified wood pulp for cotton linters in the manufacture of cellulose acetate, but the acetate solutions obtained therewith by processes which have proven suitable for the conversion .of linters have been colored and hazy and yielded products of inferior physical properties even when the starting material consisted of woodpulp having analytical constants approaching. those of purified linters. An important reason for this contrast is that the wood fibers, being much more swollen in water-saturated condition than thecotton linters due to the removal of impurities from throughout the cell wall and to the swelling. action of concentrated. alkaline solutions, undergo greater shrinkage and loss of reactivity on being dried. The conventional pretreatment with acetic acid therefore does not serve to restore conditions for contact of the cellulose with the acetylating reagents as adequately as in. the case of cotton linters.

Apparatus which is suitable for carrying out the process of this invention, as illustrated diagrammatically in the drawing, may consist of a distillation vessel I, fractionating column 5, condenser l2, receiver l4, and the necessary connections and auxiliary tanks, all parts in contact with liquid or gaseous acetic acid. being constructed of material, such as stainless steel,

which is resistant to attack by acetic acid. The

- vessel I is provided with a suitable heating jacket, not shown in the figure, perforated metal trays '2, distributing plate 3, and door 4 for charging and discharging the cellulose. Fractionatingcolumn may be the plate or packed column type. Thermometers 8 are provided at various points intheapparatus to enable control of the distillation. Condenser I2 is provided with a cooling coil or jacket (not show'n'in the figure),

and likewise receiver it if necessary, as when the distillation is carried out at reduced pressure. Tanks Sand l8 serve as storage tanks for glacial acetic acid and solvent, respectively. Vessels 6, l4, and I8 are'provided with. calibrated gauge glasses not shown in the figure, to enable the measurement of the volume 01' contained liq.- uid. i

In carrying out the process, a charge of cellu- I lose of known water content is placed on the tray 2 and the door 4 is closed and hermetically sealed.-

The required volume of glacial acetic acid is measured into the lowerpart of colinnn 5 from tank 6 through connecting pipe 1', and becomes associated with the cellulose charge after pass.-

v ing through distributing plate 3. The necessary volume of fresh solvent is measured into the bottomspace below the trays 2, of vessel I from tank Ill through connecting pipe9- or solvent condensation so that the vapor passing to condenser l2 through pipe ll contains mainly solrecovered. from a previous distillation is obtained from'receiver M by means of pipe l6.. Heat is supplied to the jacket of vessel 1 at a rate-such that the temperature readings of thermometers 8 follow a predetermined schedule. Vapor con- 'taining solvent, water, and acetic acid passes to column 5 where it is subjected to fractional vent and water vapor and only a slight amount of acetic acid. The condensate passes from condenser IZ'through pipe I3 into receiver i4, when it separates into a water layer and a solvent layer.

vent layers has collected in receiver I. The cellulose may then be transferred to the acetylator, after cooling to room temperature if desired. The aqueous layer which has collected in receiver I 4 may be removed. through pipe I! and dis.- carded o r subjected to treatment for the recovery of dissolved solvent as conditions warrant. I

If desired, the distillation may be carried'out in whole or in part at a pressure below atmospheric, vacuum line l5, supplied with suitable gauges, being provided for this purpose. When a, vacuum distillation is employed, receiver ii is cooled by means of a-jacket' or suitable cooling coil.

a The arrangement of pipes l8 and II in receiver l4, illustrated in Figure 1, is that suitable for use with a solvent having a lower density than water, and would be reversed for use with a solvent which is denser than water.

In this manner "cellulose fibers having suitable purity can be converted to a substantially anhydrous condition in a form in which they are readily reactive toward acetylating mixtures by initially supplying the fibers saturatedwith water,

- adding acetic acid to the fibers, and removing the I to vaporize by the application of heat or may; be

The completion of the distillation is indi-' cated when the proper volume oi water and solapplied to the mixture directly as' vapor.

" The starting cellulosic material may consist of purified cellulose from any-source, such as wood fibers or' cotton linters- Forthe preparation of cellulose acetate of high quality,.the purity and properties of the cellulose fibersshould be such that they are convertible, starting from the water-saturated-condition and after displacement of the water by acetic acid, by treatment with an acetylating mixture to-a useful cellulose acetate solution having low color and haze. Examples of suitable celluloses for this invention are de-' scribed in the copending applications of Lionel Goff, S. N. 113,155, filed November28, 1936; of Lionel E. Goff, et al., S. N. 81,845, filed May 26, 1936; and of Lyle Sheldon .et al., S. N. 126,362, filed February 18, 1937. A procedure for testing the suitability of a cellulose sample for the purpose of this invention will be found at the end of the specification. 1

.- The moisture content of the cellulose for treatment by the process of this invention should be between and 3 times the oven-dry weight of 25 cellulose- A desirable upper limit of moisture content can be generally stated as the residue which cannot be readily removed by pressing the waterrwet fibers. The lower limits, the least amount which yields a product'h aving a satisfactory reactivity towardacetylatin'g reagents, will generally be found to be somewhat above the fiber-saturation point.

. The amount of acetic acid to be added to the water-wet cellulose may be varied according to conditions from 3 to. 7 times the weight of oven dry material. The amount added must be at least sufiicient so that proportionate amounts of the liquids present in the mixture beforedistillation forms. homogeneous liquid phase at distillation temperature. Since the, amounts of the other liq-1 ,uids are generally fixed, as will be pointed out hereinafter, the lower limit of the amount of acetic acid which may be used is determined by the above criterion of total miscibility.

- The organic solvent to be used in the process of this invention is a liquid or mixture of liquids which is immiscible with water, totally miscble with acetic acid, has anormal boiling point preferably between and C., and forms with water-a mixture having an azetropic boiling point belowthat which tends to degrade the cellulose. Hydrocarbons, esters, and ethers having the specified properties, for example, benzene, propyl acetate and a mixture of isopropyl ether and isopropyl acetate are particularly suitable, while halogenated derivatives such .as ethylene chloride, having a tendency to hydrolyze and liberate halogen acid in the presence of water and therefore exert deleterious action on cellulose, are less solvent to be used per part of water inthe *cellu- I lose are as follows for' a number of typical solvents: benzene, 10.4; nJ-propyl .acetate,'6.0; isopropyl acetate, 9.0; and ethyl acetate, 11.2. The

composition of azeotr'opic'niixtures. can be found.

in reference ,books or readily determined-by ex periment. If a solvent'which meets the above requirements is used in the concentration of the metal-wire basket in the distillation vessel be-' dilute acetic acid solutions resulting from the acetylation process, it will generally be most convenient to utilize the same solvent in the dehydration treatment "of this invention. Preferably, an excess of solvent is used, up to or over the amount required to form the azeotropic mixture. 1 I

, The dehydration may be carried out 'in any convenient distillation apparatus, such as may be well known in the art, the portions which come into contact with acetic acid being constructed of material resistant to acetic acid. When heat is supplied to the mixture containing cellulose,

water, acetic acid, and solvent, for example, by I means of a suitable jacket, vapor which consists of solvent, water, and a small proportion of acetic acid, is evolved from. the liquid phase. By means of an appropriate fractionating column, substantially only water and solvent vapor are passedto the condenser. The distillatewhich collects separates into a solvent layer and a water layer; the former is suitable for re-use in the process while the latter may be discarded or treated for recovery of dissolved solvent, as conditions warrant. The distillation is continued until the moisture content of the cellulose is 2% or less. Small losses of acetic acid from the cellulose by distillation are of no special consequence nor is it disadvantageous to have small amounts of solvent remain in the cellulose after the treatment. By measuring the volumes of the layers of distillate which collect, it is possible to follow the course of the dehydration process and control it accordingly.

The distillation may be carried out at atmospheric or reduced pressure and at corresponding temperatures, care being taken to maintain the temperature of the cellulosebelow the order of 105 C. to avoid the degradation thereof. The time of distillation may be of the order of 4 hours or less depending on the apparatus, amount and type of cellulose being treated, and the temperature and pressure ofdistillation. However, when the optimum conditions have been determined for a given installation and material, they are maintained constant with successive lots. It

may be advantageous at times to conduct the initial part of the distillation at atmospheric pressure and complete the process at reduced pressure. Other variations are possible, for example, instead of adding all the solvent "required before the start of the distillation, portions may be added throughout the treatment.-

We have further discovered that the purity of the cellulose may be enhanced by' the process of this invention through effecting the extraction of resinous and fatty impurities during the distillation. This may be accomplished by placing the cellulose on a perforated tray or within a neath which liquid may collect during the distillation. Arrangements are preferably such that the reflux from the fractionatingcolumn is returned, to the upper surface of the massof cel-.

lulose. This returnedliquid percolates through the mass of cellulose, extracting resinous and fatty impurities, which remain as a deposit on the bottom of the distillation vessel at the end of the treatment.

The dehydration and extraction may likewise I be carried out by a procedure in which the watersaturated cellulose containing theproper quantityof acetic acid is placed on a perforated tray or and the organic solvent is initially contained in the lower portion of the distillation' vessel, out of contact with the'cellulose. Solventv vapor, which is generated on the application of heat, causes the removal of moisture from the cellulose, and the reflux liquid accomplishes the extraction of resinous and fatty impurities from the fibers, as described above.

Another important advantage of the process of this invention is'that it is readily applicable to the dehydration of cellulose fibers which have been subjected to a thorough mechanical beating operation in the presenceof water to increase the surface available to liquids. when such. beaten fibers are dried by means of hot air or vacuum drying methods, the beneficial effects of the beating treatment are offset by the increased shrinkage and bonding of the fibers and fiber elements, such as occur during the manufacture of paper, and for that reason, it has been desirable to avoidany beating of cellulose fibers intended for use in the conventional acetylation processes. Likewise, it is not practical to dehydrate well-beaten fibers by displacement with a liquid column of acetic acid due to the greatly,

increased pressures required compared to a similar process applied to unbeaten fibers.

In order more particularly to describe the present inventio1 1.-there follows the description of typical embodiments of the. invention. .It' will be understood that the various features set forth in connection with these embodiments are by way of illustration only and may be considerably varied the scope of the present invention.

Example A Purifiedwood cellulose, for example, that pre pared from unbleached spruce sulfite pulp by the process describedin the copending application of Lionel Goff, S. N. 113,155, containing, for example, 98.6% alpha cellulose, 2 6% of soda solually raised during a 2 hour period to 80 C. and v the pressure reduced to the order of half an atmosphere, while the distillation is continued. Finally, with the temperature at 80 C., the pressure is reduced to one-third anatmosphere for a shorttime in order to complete the treatment. The pulp at this stage will show substantially no change in the usual analytical constants excepting a slight reduction in cuprammonium viscosity, of the order of 1 or 2 centipoises, and a lower content of ether-extractive material.

The acetylation of the dehydrated pulp may then be carried out, for example, by the process described in the copending application of Lyle Y Sheldon et al., s. N. 70,372, fiied March 23, 1936,

which consists briefly of cooling the cellulose-to 25 C., treating at that temperature with a mixture of 2.45 parts of 95% acetic anhydride and 0.026 part of sulfuric acid per part of cellulose, and completing the reaction by allowing a gradual rise in temperature to 42.5? C. The acetate may then be precipitated or converted to the acetone-soluble form and further treated by methods known in the art.

ErmgleB Cellulose, such as that described in Example A, containing 1.75 times its weight of moisture, is mixed with 5 parts of acetic acid, and subjected to dehydration with the use of 12.5 parts of normal propyl acetate per part of cellulose as described in the above example excepting that the distillation is carried out at atmospheric pressure and the maximum temperature to which the cellulose is heated is below 105 C. After substantial removal of the moisture has been eifected, the acetylation and further treatment may be carried out as described in Example A. y

We have thus disclosed a process for rendering cellulose readily and economically convertible to cellulose acetate by means of which the manufacturing cost from conventional raw materials may be reduced and less expensive raw materials made available for conversion which have heretofore been unsuitable. The basic features of the said process consist in starting with cellulose which is saturated with water, adding acetic acid to the cellulose, and volatilizing the moisture with the vapor of a volatile organic solvent, which in the liquid state is totally miscible with acetic acid and imm' cible with water, under conditions as specifii, which avoid shrinkage and degradation of t cellulose and efiectan improvement in the reactivity and purity thereof.

Cellulose which has been treated by the process of this invention is obtained in fiufly form, saturated with acetic acid, containing less than 2% moisture, and in a state of swelling substantially corresponding to that of its initial watersaturated condition. It is highly responsive to treatment with aceylation mixtures, yielding useful cellulose actate solutions at lowered cost. Acetic acid in the process may be replaced in whole or in part by the other low molecular weight fatty acids which are totally miscible with and less volatile than water, such as propicnic and butyrio acids. Such substitution is particularly useful when the cellulose is tobe converted to the propionate or butyrate, or mixed esters. Formic acid while a low molecular weight fatty acid totally miscible with water, is not uniformly less volatile than water and is not suitable for use in the process of this invention (International Critical Tables, vol. pages 212 and 215).

As many changes could be made in carrying out the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

In the foregoing specification .and following claims, terms have been used having the following meanings:

Alpha cellulose is defined as that portion of a sample of cellulosic material not dissolved by 17.5% sodium hydroxide solution at 20 C., determined by a refinement of the method described by H. F. Lewis in "Technical Association Papers, series XVII, #1 436 (1934). A

Soda soluble material is defined as that portion of a cellulosicsample dissolved when it is subjected to the action of 7.14% sodium hydroxide solution at the boiling point of water for 3 hours by a refinement of the method of Griffin,

"Technical Methods of Analysis," 492 (1927 ed.) cuprammonium viscosity, as used herein,

means the viscosity number or value obtained according to the following method: The cuprammonium solution is prepared by the action of air on electrolytic copper in the presence of strong ammonia water. The copper concentration of the solutions employed for-viscosity determinations is 30, :2 g. per liter and the ammonia content is 165 g., :2 g. per liter. The concentration of cellulose employed is 0.6 g. (oven dry basis) per 100 cc. of cuprammonium solution. The cellulose sample for this determination is dried at 70 C. to 4% moisture, content. After weighing out 0.6 g. (oven dry basis), the

sample is moistened, squeezed to a weight of 2 I g. and then dispersed in cuprammonium solution in an atmosphere of hydrogen from which oxygen has been completely removed. The viscosity measurements are made at C. with a modified Ostwald pipet, constructed according to the specifications of the Americal Chemical Society Committee on the Viscosity of Cellulose (Journal. of

Industrial 8; Engineering Chemistry, I, #49; 1929) The time of flow in seconds is converted to centipoises on the .basis of the calibration of the pipet with oils of known viscosity in centipoises obtained from the UnitedStates Bureau of Standards.

Method for testing suitability of cellulose for acetylation.A 10 g. sample is agitated in times its weight of water at room temperature for 15 minutes and then drained and pressed free of excess water. The operation is repeated four times using glacial. acetic acid instead of water. The drained and pressed cake finally obtained is weighed, placed in a suitable bottle in shredded condition, and brought to a temperature of 25 C. Sufiicient glacial acetic acid, containing.0.36

' g. of 96% sulfuric acid, is added to bring the total amount of acetic acid to '70 g. After thorough mixing and allowing the mixture to stand for 15 minutes at 25 C., 301grams of acetic anhydride are incorporated into the mass and the mixture maintained at 40 C. for 30 minutes with occasional stirring, when the reaction is stopped by adding 15 c. c. of 70% acetic acid. After entrapped air bubbles have been removed by applying suction and the mixture has been allowed to stand for 1 hour at 25 C., the color and haze are measured and expressedin terms of the standards of the American Public Health Association described in "Standard Methods of Water Analysis, pp. 4-11 (1933 ed.)-. By the above procedure, a good quality of acetylatible cellulose will yield a reaction mixture having about 50 parts per million color and haze.

We claim: v

1. The method of conditioning cellulose for chemical treatment comprising providing purified cellulose with its fibers in swollen condition and saturated with water, subjecting the said water-saturated cellulose to distillation in the presence of a low molecular weight fattyacid which is totally miscible with and uniformly less volatile than water such as acetic, propionic or butyric, and of an organic solvent which is miscible with and more volatile than the said acid and immiscible with water, said combined liquids forming a homogeneous liquid phase at distillation temperatures, and continuing the distillation until the cellulose is substantially free of water including reaction with the cellulose in the pres- .ence of a low molecular weight fatty acid totally miscible with and uniformly less volatile than water such as acetic, propionic or butyric, the

method of preparing cellulose for 'such reaction inswollen condition of its fibers substantially satuated with said acid and free from water which comprises providing water-swollen wood cellulose, and replacing the water with said acid while maintaining the cellulose fibers in swollen condition by subjecting the saturated cellulose in the presence of said acid to distillation of the water with an organic solvent which is immiscible with water, miscible with and more volatile than the said acid, and which forms an azeotropic mixture with the water having a boiling 3. In the manufacture of a cellulose derivative I including reaction with cellulose in the presence '01 a low molecular weight fatty acid totally miscible with and uniformly less volatile than water such as acetic, propionic or butyric, the method of preparing cellulose for such reaction in swollen condition substantially saturated with said acid and free from water which comprises providing water-swollen wood cellulose and replacing the water in the cellulose with said acid while maintaining the cellulose in' swollen condition, by subjecting the water-saturated cellulose to distillation in the presence of said acid with an organic solvent which isimmiscible with water, miscible with said acid, and has a normal boiling point between 75 C. and 105 C'.,'the acid, solvent, and water forming a homogeneous liquid phase continuing said distillation to free said cellulose of said water and solvent and saturate said cellulose with acid in said swollen condition of the cellulose fibers, and then treating said cellulose to form said derivative.

4. In the manufacture of cellulose acetate the method of preparing cellulose in swollen condition substantially saturated" with acetic acid and with acetic acid while maintaining the cellulose in swollen condition by subjecting the saturated a cellulose and acetic acid to distillation with a I volatile organic solvent which isimmiscible with water and misciblewith acetic acid, has a normal boiling point between 75 C. and 105 C., and forms a homogeneous liquid phase with the water and acetic acid continuing said distillation to free said cellulose of said water and solvent and saturate said cellulose with said acid in said swollen condition of the cellulose fibers, and then treating said cellulose to form said acetate.

5. In the manufacture of cellulose acetate,the

method of preparing cellulose in swollen condition substantially saturated with acetic acid and free from water which comprises providing cellulose maintained. in swollen condition by saturation with water, and replacing the water with acetic acid while maintaining the cellulose in swollen condition by subjecting the saturated cellulose in the presence of, acetic acid to distillation with an organic solvent of the group consisting of benzene, propyl acetate, isopropyl acetateand isopropyl ether, or mixtures of any of them, the said solvent forming with the acetic acid and water a homogeneous liquid phase, continuing said distillation to free said cellulose of said water and solvent andsaturate said cellulose with said acid in said swollen condition of the cellulose fibers, andthen treating said cellulose to form said acetate.

6. In the 'manufacture'of cellulose acetate the process of preparing cellulose for acetylation which comprises removing water from the cellulose in the presence of acetic acid by distillation with a volatileorganic solvent immiscible with water, miscible with acetic acid, and having a boiling point below that of acetic acid, subjecting the vapors to fractional condensation, and extracting soluble impurities from the cellulose by. percolating the reflux liquid through the cel- -'lulose.

I 7. The method of conditioning cellulose for water, the combined liquid forming a homogeneous liquid phase, subjecting the liquids to fractional condensation and extracting soluble im-- purities from said cellulose by percolating the reflux liquid through said cellulose and continutially free of water and solvent.

- JOSEPH FLEISCHER.

WIILIAMABAEEY TRQ 

